Then gnome and gnome-core packages are meta-packages, meaning that they exist only to depend on other packages making it easier to install a complete set of packages. Here iceweasel is considered to be part of the set and so it is included as a dependency. So removing those packages will not remove anything important.
But, if those packages are not installed, when you upgrade your system any new Gnome components that would have been pulled in by those meta packages will not be installed. This still shouldn't cause anything to break since individual packages will have dependencies on the packages that they require; but it would mean that you would no longer have a complete Gnome environment (minus iceweasel).